This application relates to and claims priority to corresponding Japanese Patent Application No. 313462/1999, which was filed on Nov. 4, 1999, and which is incorporated by reference herein.
(1) Field of the Invention
The present invention relates to a generator which is driven by an engine (hereinafter referred to as an xe2x80x9cengine-driven generatorxe2x80x9d), and more particularly to an engine-driven generator having improved starting characteristics and requiring no control of the driving engine for starting the load wherein the heavy load is loaded on the output of the generator, for example, the load of the induction motor.
(2) Description of the Prior Art
In the case when an induction motor as a load is started by an engine-driven generator, since there flows a rush-current in the order of 400-800% of the rated current thereof at the starting, it is necessary that the capacity of the generator be two-three times as that of the induction motor, so that the performances of the generator are not sufficient for such case. In other words, if the generator having substantially the same rated capacity as that of the load is used, the revolution number of the driving engine is greatly lowered, or the engine will stop its operation.
In order to improve the above explained problems, conventionally, the following methods by which the input supplied to the induction motor is limited are taken.
(1) At starting, the output voltage of the generator is lowered and the input to the induction motor is limited accordingly (method of reducing voltage).
(2) In addition to the above reducing voltage method, the revolution number of the driving engine is temporarily lowered by the control operation of the governor (i.e., reduction in the supply amount of the fuel). Or, by utilizing the temporary reduction in the revolution number of the driving engine due to the rush current, the slip of the induction motor at the starting is made small, whereby the input supplied to the induction motor is limited to low with the sufficient torque being maintained.
(3) By utilizing an automatic voltage regulating means as disclosed in the Japanese Patent No. 2889972, the revolution number of the engine is temporarily lowered at the time when the load current is high, a voltage which is proportional to the revolution number of the engine is outputted by the automatic voltage regulating means, and the revolution number of the engine is gradually raised up to the rated number of revolution accordingly.
FIG. 9 is a block diagram of an exemplary construction of a device that includes an automatic voltage regulating means in which A denotes an engine, B denotes a generator, C denotes a motor, D denotes an automatic voltage regulator, E denotes a control selection means, F denotes a load current detection means, and G denotes a speed control means.
FIG. 10 is a more detailed block diagram of an exemplary construction of a device that includes an exemplary automatic voltage regulating means in which numeral 1 shows an engine, numeral 2 shows a generator which is driven by the engine 1, and numeral 3 shows an induction motor powered by the output of the generator 2. The induction motor 3 is connected to the generator 2 through a load switch 4 and a breaker 5. Numeral 6 shows an automatic voltage controller, numeral 7 shows a current transformer which is provided at an output circuit of the generator 2, numeral 8 shows a fuel pump for the engine 1, numeral 9 shows an actuator for the fuel pump 8, numeral 10 shows a speed sensor in combination with a rotation gear 10a, for detecting the speed of the engine 1, numeral 11 shows a control selection switch, and numeral 12 shows a speed controller.
The automatic voltage regulator 6 maintains a rated voltage (for example, 220 volts) where the frequencies are within predetermined ranges higher and lower than the rated frequency (for example, 60 Hz). However, the frequency becomes lower than the above predetermined range, the output voltage lowers. FIG. 11 shows exemplary characteristics of such regulator. As shown in a solid line, below a predetermined frequency, the voltage becomes a certain voltage which is set lower than the rated voltage, for example, 50% of the rated voltage. Alternatively, the voltage decreases continuously in proportional to the decrease of the frequency as shown in a dotted line.
The above method (1) is effective for such load having small starting inertia as a pump or a fan. Though the input to such load becomes small proportional to the square of the voltage, the torque generated also becomes small in proportional to the square of the voltage. Therefore, the method can be used only for the case where the load is small at the starting, however, for loads other than such load, it is necessary that the generator having enough capacity be selected as in the conventional way.
In the above method (2), as the input is decreased as the induction motor is accelerated, the revolution number of the engine rises. The motor must be accelerated in the state in which the slip is constantly large and, as a consequence, a prolonged time is required before the motor is accelerated up to the rated revolution number and, during this time, an excess current flows so that the generator requires the capacity such that the excess current breaker does not operate in the course of starting the motor.
With the method (3), the starting is easy because, as compared with conventional systems, the output voltage is proportionally controlled by the revolution number of the motor and the automatic voltage regulating means. However, since the revolution number of the engine is changed in a plurality of steps, it requires a fuel control section for precisely controlling the revolution number of the engine, which leads to a high cost of the entire system. Also, since the revolution number of the engine is caused to be lowered, the output of the engine with respect to the torque required by the load side is inevitably lowered.
In view of the foregoing, an object of the invention is to improve the load capacity which enables the starting by the engine-driven generator without directly controlling the engine and to realize a method of starting an engine-driven generator at a low cost and to provide the engine-driven generator without requiring complicated costly equipment. Also, since the starting time is, in any case, limited by V/F constant characteristics, in order to shorten further the starting time or to increase the load capacity by causing the starting current to be lowered, there is no alternative but to increase the capacity of the generator.
As a means to solve the above problem, the present invention provides a method for starting the load of the engine-driven generator and provides the engine-driven generator which controls the generator excitation current such that, at the starting of the motor to which the power is supplied by the engine generator, when the generator output voltage has become lower than a predetermined voltage, the output frequency of the generator approaches the predetermined value lower than the rated frequency, and subsequently when the generator output voltage has arisen to the second predetermined voltage which is higher than the first predetermined voltage, the generator output voltage becomes the rated voltage.
According to the invention, in order to start the motor which becomes the heavy load with respect to the generator capacity, when after the starting, the output voltage (for example, rated 220 volts) of the generator has become lower than the first predetermined voltage (for example, 200 volts) determined in advance, the revolution number of the engine is such as to be lowered to the predetermined number by the control of the generator excitation current. The generator excitation current is controlled so that the load of the load of the generator is raised and, since the revolution number of the engine rises due to the increase in the load, the revolution number of the generator driven thereby is lowered. Specifically, the generator load is increased by the increase in the excitation current and, by lowering the revolution number of the driving engine, the excitation current is controlled so that the generator output frequency (for example, 60 Hz) becomes the predetermined frequency (for example, 33 Hz). That is, the control is made for the excitation current to be increased in order to increase the generator load. By controlling the excitation current so as to approach the predetermined frequency, the motor torque can be maintained while the input current is suppressed to low in the state in which the output frequency of the generator is low and the slip of the motor at the starting is small, thereby enabling the smooth starting and the enhancement of starting characteristics.
In the foregoing, when the excitation current is increased, the revolution number of the engine is lowered. However, unlike under the prior art, the lowering of the revolution number of the engine is not performed by restricting the fuel, that is, no fuel control is performed. That is, the output torque characteristic curve of the engine is not changed. The output torque is changed along the lowering of the revolution number following such torque characteristic. The fact that the torque characteristic of that time is of larger output torque, at the same revolution number, than the output torque in which the revolution number is lowered by restricting the fuel, is clear from the change in the torque characteristic curve with respect to the fuel supplying amount of the engine.
After the starting, while the excitation current mentioned above is being controlled, the revolution of the motor rotor rises and, with this rise, the starting current is lowered and the generator output voltage rises. When generator output voltage rises to the predetermined voltage, the excitation current is controlled so that the generator output voltage becomes the rated voltage (for example, 220 volts). Here, it can be considered that the excitation current be controlled following the conventional V/F constant characteristic starting from the time when the output voltage has become 95%, for example, 209 volts with respect to the rated 220 volts. According to the invention, since the output voltage is larger as compared with the voltage value with respect to the frequency at the V/F constant characteristic, it has been found that there appears a phenomenon in which the output voltage once lowers down by the switching. Thus, the finding is that, when the output voltage is raised to about 95% while the excitation current is controlled so as to be approached to a low frequency, and the control of the excitation current is switched so as to become the rated voltage, the rated frequency and the rated voltage can be ensured to be established.
Further, according to the invention, in the course of making the generator output voltage to the rated voltage, when the detection is made that the output frequency of the generator has become the rated frequency (for example, 60 Hz), the control is shifted to the control of the generator excitation current based on the FV characteristics by the output voltage and the frequency.
The term xe2x80x9cFV characteristicsxe2x80x9d here refers to the so-called xe2x80x9cAVRxe2x80x9d control which covers the V/F constant characteristics at the low frequency region and the constant voltage characteristics at the high frequency region. According to the invention, after the rated voltage is once established at the starting, the control is shifted to that for AVR control following the general FV characteristics.
As above, at the starting of the motor, the excitation of the generator is increased and the braking action is exerted to the generator and, as a consequence, the revolution number of the engine is controlled so that a high cost equipment for making a fuel control is unnecessary. Also, since the fuel control is not relied upon, the output torque characteristic curve of the engine does not change so that, even when the number of revolution of the engine is lowered, the generator can be driven without sudden lowering of the output torque. Further, even when the load to the generator is increased, the generator is driven firmly by the engine which does not undergo the sudden lowering of the output torque, and it is possible to elevate the limit at which the engine stops due to an excessively high load, and to enhance the load capacity by which the generator can carry the load.
At the starting, the excitation current is increased and the output frequency of the generator is lowered so that the motor torque is maintained while the slip of the motor is made small and, by suppressing the input current, the raising of the revolution number of the motor to the predetermined number is smoothly carried out. Thus, the starting current does not flow for an extended time and, since the excess current can be prevented, the starting with a heavy load is enabled.
According to the invention, each of the various parts concerned is controlled by relying not on the monitoring of the load current but on the monitoring of the progress in the rising of the output voltage of the generator. That is, the subject covered by the invention is different from the subject in which the control is made by detecting the load current.